Cell structure for heat exchanger



April 20, 1965 E. MAILLET CELL STRUCTURE FOR HEAT EXCHANGER 3 Sheets-Sheet 1 Filed Jan. 21, 1963 T INVENTOR.

"Elva/Emma, MH/LLET HTTGRNEYS April 20, 1965 E. MAILLET CELL STRUCTURE FOR HEAT EXCHANGER Filed Jan. 21, 1963 3 Sheets-Sheet 2 INVENTOR [AWE/MONO fife/LL57- BY 6 TTOP/VE Y5 E. MAILLET CELL STRUCTURE FOR HEAT EXCHANGER,

3 Sheets-Sheet 3 April 20, 1965 Filed Jan. 21, 1963 United States Patent ()fiice 3,179,573 CELL STRUCTURE FOR IEAT EXQHANGER Ennemond Maillet, Paris, France, assignor to Commissariat a lEnergie Atomique, Paris, France Filed Jan. 21, 1963, Ser. No. 252,835 Claims priority, application France, Feb. 8, 1962, 887,434 5 Claims. (Cl. 176-453) This invention relates to a tubular element cell for a heat exchanger wherein the transfer of heat from the cell is promoted by the provision of a liquid film maintained on the inner wall of the elements, in contact with an axial flow of vapour of the same fluid.

This invention is of particular interest for a heat transfer cell of the type used as a fuel cell in a nuclear reactor to permit the direct feeding of a steam turbine under adequate pressure and temperature conditions.

It is known that in a heat transfer tube the coefiicient of heat transfer between the fluid and the tube wall can be improved considerably by causing liquid droplets to be conveyed at a relatively high speed in a central stream of gas or vapour, this improvement attended by an increase in the resulting turbulence of the liquid film being caused notably by the partial vaporization of the liquid phase. However, it is observed that the thinning down of the liquid film as a consequence of the considerable speed necessary for circulating the gaseous phase reduces the thermal flux likely to be supported without any risk of destroying the tube material, this phenomenon being generally called burnout. Under these conditions to preserve the cooling capacity of the film, it is necessary to use a relatively high degree of latent vaporization heat, thus limiting the vapour pressure and the efficiency of the turbine associated with the plant.

It is the essential object of the present invention to avoid the drawbacks listed hereinabove by providing a simple yet eflicient arrangement whereby a minimum liquid-film thickness can be maintained even with a strongly accelerated vapour flow.

To this end, a tubular element cell for a heat exchanger according to this invention is characterized notably in that it comprises an external liner divided into two portions of substantially the same length by an intermediate partition extending at right angles to the cell axis and intermediate the cell ends, each one of said two portions comprising at least one inlet duct for a cooling fluid in the liquid phase and a plurality of tubular elements for vaporizing said fluid which are disposed symmetrically at spaced intervals about the axis of said cell, said inlet duct and said vaporization elements being connected in the vicinity of said intermediate partition by means of radial ducts having a nozzle-like tapered configuration adapted on the one hand to throttle said liquid phase and on the other handto form a liquid film on the inner wall of said vaporization elements, said film flowing in the axial direction with a helical motion along the walls of said elements.

In addition to this main arrangement the invention further provides various secondary arrangements to be set forth in detail presently as the description of a typical form of embodiment of the invention proceeds with reference to the accompanying drawings. In the drawings:

FIGURE 1 is a fragmentary longitudinal axial section showing a tubular cell element for a heat exchanger, which is constructed according to the teachings of this invention;

FIGURE 2 illustrates at the right side thereof a crosssectional view of the cell shown in FIG. 1 and at the left side thereof a diagrammatic sectional view of a radial duct for connecting the cooling fluid inlet duct to one of the tubular vaporization elements;

3,l7,573 Patented Apr. 20, 1965 FIGURE 3 is a first diagram showing the defective position of the heat flux curve in relation to the burnout limit flux curves in a heat exchanger tubular cell of known type;

FIGURE 4 is another diagram showing the curve setting obtaining with a central injection arrangement according to the present invention in the case of the cell illustrated in FIG. 1;

FIGURE 5 is a complementary diagram showing clearly the improvement resulting from the injection of the liquid phase into the cell with sustained rotation of the fluid film along the inner walls of the tubular vaporization elements;

FIGURE 6 illustrates a modified form of embodiment of a cell according to this invention, and

FIGURE 7 is a cross-sectional view of the cell shown in FIG. 6.

In a first form of embodiment illustrated in FIG. 1 a tubular cell element constructed according to this invention comprises an outer liner 1 extending coaxially within a protection sheath or insulating sleeve 2, the gap between the two elements being utilized if desired, for disposing therein any heatinsulating material or a stagnant sheet of neutral gas ensuring a convenient heat insulation of the cell relative to the external atmosphere.

The liner 1 is divided into two portions of substantially equal length by an intermediate partition 3 secured in a fluid-tight manner on the liner 1 and reinforced on either side by rigid plates such as 3:: imparting a suitable mechanical strength thereto. Each portion of the liner 1 is hand arranged in the same manner and comprises a longitudinal central duct 4 and a series of longitudinal planetary tubular elements 5 disposed on spaced radii about the central duct 4. Central duct 4 and the various tubular elements 5 are supported by a grid 6 divided into radial ducts 7 providing separate fluid connections between each planetary tubular element 5 and the central duct 4. These radial ducts have an inner configuration tapering toward their junction with the relevant tubular elements 5 (see left-hand portion of FIG. 2) and merge tangentially into these elements 5.

In the specific example illustrated in FIGS. 1 and 2 the tubular cell is a nuclear fuel cell adapted for use in a power tube of an atomic reactor. To this end, the fissile material is in the form of annular elements 8 surrounding the tubular elements 5, the gap formed between these tubular elements 5 and the central duct 4 being filled if desired with a low-absorption or moderator substance 9 such as graphite.

During the reactor operation the heat developed during the fuel fission reaction is transferred as follows: a cooling fluid in the liquid phase is introduced into the central duct 4 under adequate predetermined pressure and output conditions. To prevent the cooling fluid from vaporizing in this zone the liquid is injected under a relatively high pressure and its path is shown by the arrows 10, FIG. 1. It will be seen that the fluid is injected at the base or inlet end of each planetary tubular element 5 after the liquid phase has been throttled in the neck portion 7a of the relevant tapering radial duct 7. Moreover, as the injection takes place tangentially to the particular element 5, a movement of rotation is imparted to the liquid film formed on the inner walls of these tubes (which is illustrated in the form of broken lines Illa), this movement of rotation promoting the heat transfer coeflicient as will be explained hereinafter.

Thus, the above-described arrangement ensures, within the tubular outlet elements 5 and under a pressure value inferior to that maintained in the central duct 4, a convenient vaporization of the liquid portion of the mixture due to the strong expansion produced in the radial tapered ducts 7, while constantly maintaining a rotating liquid film along the tube walls. n the other hand, by supplying to and circulating fluid in the feed duct 4, a kind of thermal regulation is applied to the cell, the liquid injection constantly occurring a few degrees below the saturtion temperature, thus aifording a constant control of the vapour rate at the inlet end of the planetary tubular elements 5.

The diagram of FIG. 3 shows in ordinates the heat flux curves and in abscissa the vapour rate produced in the tubular elements of a nuclear fuel cell. The curves gob are traced for mass outputs ranging from 2.85 lbs/sq. in./s. (tpb to 7.1 lbs/sq. in./s. 12 and correspond to the limit or burnout flux curves below which the heat flux curve corresponding to the heat transfer produced in the cell should lie. It will be seen that, in order to take due account of a suitable safety margin, the reactor flux gar is strongly limited, this accounting for the corresponding reduction in the heat transfer rate.

FIG. 4 illustrates the adaptation of the diagram of the preceding figure to the specific case of a heat exchanger cell of the type shown in FIGS. 1 and 2, according to the teachings of this invention. Thus, it is clear that by injecting the liquid into the central or intermediate portion of the cell the corresponding curves (pb can be divided into two and provides a substantial gain in the permissible height and width of curve r. Moreover, by sustaining the gyratory motion of the two-phase flow by means of any suitable arrangement such as helical fins or blades, for example, disposed within the tubular elements, the contact between the liquid film and the wall may be improved. As a result, the limit flux or burnout curve gel) is brought back to t 'b as shown in FIG. 5, thus increasing the possibilities for establishing the curve g0) and providing, for a same vapour rate at the outlet of the tubular element which may be as high as .6 to .8, a convenient safety of operation.

Of course, the invention should not be construed as being limited to the form of embodiment shown and described herein by way of example, as many modifications and alterations may be brought thereto without departing from the spirit and scope of the invention as set forth in the appended claims. Thus, the liquid may be introduced only by means of a central inlet duct, by means of separate tubes, by a header or a central liquid sheet, or by any other suitable means. Thus, when the annular fuel elements 8 are in the ceramic form such as U0 for instance, it is advantageous to provide not only a cooling from inside as shown in FIGS. 1 and 2, but also an external cooling. Under these condition the cooling fluid may be directed to the central or intermediate portion of the cell through interstices formed between the elements, the cross-sectional area of these interstices being reduced if desired by introducing therein a certain thickness of suitable absorbent or moderator material to keep the liquid phase output at a satisfactory limit value. In the alternative embodiment of FIGS. 6 and 7 corresponding numerals distinguished by the addition of the letter a have been employed to indicate parts that are the same as the corresponding elements in the embodiment of FIGS. 1 and 2 or which are closely analogous thereto. FIG. 6 illustrates a cell of this character wherein the external passages such as 11a of relatively small cross sectional area are intended for introducing the liquid phase at a temperature below the saturation value, the inner passages 12a of greater cross-sectional area being used for the low-density moisture-laden vapour produced as in the preceding example during its travel to the exterior of the cell. FIG. 7 illustrates a cross section of this modified arrangement.

It will be readily understood by anybody conversant with the art that any other configuration or structure of fuel elements capable of bringing the fluid in liquid phase to the center of the reactor and of vaporizing this fluid by evacuating the same in the form of mist with a controlled fiow raising the burnout limit by maintaining the liquid phase in contact with the heating walls, would meet the basic requirement of this invention. This would apply notably to clusters of fuel rods wherein the vapour path is directed through the liquid-vapour mixture by means of deflector or spiral members ensuring a helical fiow with central vapour cores.

What I claim is:

1. A cell structure for a nuclear reactor heat exchanger, comprising: an outer liner divided into two portions of substantially equal length by an intermcdaite partition extending at right angles with respect to the axis of said liner in the intermediate portion thereof, each of said portions comprising at least one inlet duct extending in an axial direction within said liner for the admission of cooling fluid in the liquid phase and a plurality of tubular elements for vaporizing said fluid, said tubular elements being disposed in an axial direction and in spaced symmetrical relation within said liner, and throttling means disposed in the vicinity of said intermediate partition connecting the inlet duct in each liner portion to each of said tubular elements therein, said throttling means comprising a plurality of throttling ducts tapering inwardly in the direction of flow from said inlet ducts to said tubular elements, respectively, to thereby throttle said liquid phase, said ducts communicating with said tubular elements in a tangential direction to thereby form a liquid film on the internal walls of said tubular elements which flows in a generally helical pattern passing axially through said tubular elements.

2. A cell structure as defined in claim 1 wherein a single, centrally disposed inlet duct is provided within each of said portions, said ducts extending coaxially through the respective portions and being connected at their adjacent ends to said throttling ducts, said throttling ducts extending radially outward from said inlet ducts and being connected to tubular elements disposed symmetrically within said liner portions and spaced radially outward from the respective inlet ducts.

3. A cell structure as defined in claim 1 wherein annular elements of fissile material are provided in surrounding relation with respect to said tubular elements.

4. A cell structure as defined in claim 1 wherein said tubular elements are disposed symmetrically among cylindrical elements filled with fissile material and disposed parallel to the axis of said cell.

5. A cell structure as defined in claim 3 wherein moderator material is provided in the interstices between said inlet duct, said tubular elements and said annular elements of fissile material.

References Cited by the Examiner UNITED STATES PATENTS 3,063,925 11/62 Huet 176-83 3,070,537 12/62 Treshow 17678 3,087,881 4/63 Treshow 176-54 3,104,219 9/63 Sulzer 176-78 FOREIGN PATENTS 1,197,317 6/59 France.

882,598 11/61 Great Britain.

CARL D. QUARFORTH, Primary Examiner, 

1. A CELL STRUCTURE FOR A NUCLEAR REACTOR HEAT EXCHANGER, COMPRISING: AN OUTER LINER DIVIDED INTO TWO PORTIONS OF SUBSTANTIALLY EQUAL LENGTH BY AN INTERMEDAITE PARTITION EXTENDING AT RIGHT ANGLES WITH RESPECT TO THE AXIS OF SAID LINER IN THE INTERMEDIATE PORTION THEREOF, EACH OF SAID PORTIONS COMPRISING AT LEAST ONE INLET DUCT EXTENDING IN AN AXIAL DIRECTION WITHIN SAID LINER FOR THE ADMISSION OF COOLING FLUID IN THE LIQUID PHASE AND A PLURALTIY OF TUBULAR ELEMENTS FOR VAPORIZING SAID FLUID, SAID TUBULAR ELEMENTS BEING DISPOSED IN AN AXIAL DIRECTION AND IN SPACED SYMMETRICAL RELATION WITHIN SAID LINER, AND THROTTLING MEANS DISPOSED IN THE VICINITY OF SAID INTERMEDIATE PARTITION CONNECTING THE INLET DUCT IN EACH LINER PORTION TO EACH OF SAID TUBULAR ELEMENTS THEREIN, SAID THROTTLING MEANS COMPRISING A PLURALTIY OF THROTTLING DUCTS TAPERING INWARDLY IN THE DIRECTION OF FLOW FROM SAID INLET DUCTS TO SAID TUBULAR ELEMENTS, RESPECTIVELY, TO THEREBY THROTTLE SAID LIQUID PHASE, SAID DUCTS COMMUNICATING WITH SAID TUBULAR ELEMENTS IN A TANGENTIAL DIRECTION TO THEREBY FORM A LIQUID FILM ON THE INTERNAL WALLS OF SAID TUBULAR ELEMENTS WHICH FLOWS IN A GENERALLY HELICAL PATTERN PASSING AXIALLY THROUGH SAID TUBULAR ELEMENTS. 